The present invention relates to a silver halide photographic emulsion, a method of manufacturing the emulsion, and a method of inhibiting aggregation of the emulsion. The present invention particularly relates to a silver halide photographic emulsion which is spectrally sensitized by adding a cyanine dye and in which tabular grains having an aspect ratio of 3 or more account for 50% or more of the total projected area, a method of manufacturing the emulsion, and a method of inhibiting aggregation of the emulsion.
Silver halide photographic emulsions are generally manufactured through grain formation, desalting, spectral sensitization, and chemical sensitization. To improve the sensitivity/graininess ratio of a silver halide photographic emulsion, spectral sensitization using a sensitizing dye is recently generally performed by adding the sensitizing dye during or before chemical sensitization, as disclosed in, e.g., U.S. Pat. No. 4,433,048.
In this spectral sensitization performed by adding a sensitizing dye during or before chemical sensitization, the sensitivity/graininess ratio greatly improves when the amount of sensitizing dye to be added is 50% or more, preferably 60% or more of the saturated covering ratio of silver halide photographic emulsion grains. This is described in Jpn. Pat. Application No. 5-145355, whose Jpn. Pat. Appln. KOKAI Publication number (hereinafter referred to as JP-A) is 6-332091.
When a particularly large amount of a sensitizing dye is used, however, the sensitivity/graininess ratio is improved and at the same time the photographic properties degrade due to aggregation of emulsion grains, as disclosed in JP-A-6-332091. Silver halide emulsion grains aggregate especially in a tabular silver halide photographic emulsion in which tabular grains having an aspect ratio of 3 or more account for 50% or more of the total projected area. It is considered that this aggregation of silver halide emulsion grains takes place because a sensitizing dye adsorbed at a high covering ratio makes gelatin lose its protective colloidal properties for the silver halide emulsion grains. When aggregation occurs, the photographic properties degrade, e.g., the sensitivity lowers, the fog rises, and the graininess degrades. Additionally, coarse grains makes the manufacture difficult to perform. Accordingly, it is being strongly desired to solve this problem of aggregation.
JP-A-6-332091 has disclosed that addition of a silver iodobromide fine grain emulsion can solve the problem of aggregation of tabular silver halide grains when a large amount of a sensitizing dye is added. However, the aggregation inhibiting effect of calcium or magnesium in an emulsion is entirely unknown.
EP 590,725 has disclosed a method of manufacturing an emulsion in the presence of a water-soluble metal salt such as calcium nitrate or magnesium sulfate. However, although the solution concentration when the metal salt is added is described, the metal salt content in an emulsion is not specifically described. Also, the saturated covering ratio of sensitizing dye is not described. Additionally, EP 590,725 does not refer to aggregation of tabular silver halide grains which is a problem when a large amount of a sensitizing dye is added. Therefore, EP 590,725 is evidently different from the present invention.
JP-A-3-174142 has disclosed a method of manufacturing a high-speed silver halide sensitive material having high storage stability with time obtained by adjusting the calcium content of gelatin in the material. However, although the content in gelatin is specifically described, the content in an emulsion is not described. Also, aggregation of a tabular silver halide emulsion is not referred to. Furthermore, JP-A-8-272021 describes the calcium concentration of coating solution containing tabular grains. However, calcium is added to the solution to be coated. That is, JP-A-8-272021 does not describe any addition during the course of manufacture of a spectrally sensitized tabular silver halide emulsion.
That is, no method is known which uses calcium or magnesium to eliminate aggregation of tabular silver halide grains which is a problem when a large amount of a sensitizing dye is added.